Printed circuit boards or printed wiring boards (PWB) comprise an insulative substrate upon which a metal pattern defining an electronic circuit has been formed. Electronic devices are typically mounted on the PWB to form an electronic system or subsystem. As described, for example, in the book Printed Circuits Handbook, F. Coombs, Jr., McGraw Hill (1979), the metal pattern defining the circuit may be made by procedures known as the subtractive, the additive, or the semi-additive process.
With the subtractive process, a metal layer is bonded to the surface of the insulative substrate by a procedure such as the hot press lamination process. A photoresist is then coated over the metal layer and exposed through a patterned mask to actinic radiation. After development of the photoresist, the metal layer is selectively etched to leave a metal pattern defining the conductive pattern desired.
With the additive process, a patterned plating mask is used to mask certain regions of the insulative substrate as metal is being plated on exposed portions of the substrate. After proper preparation of the surface of the substrate (as by catalyzation), the metal can be plated directly on it by electroless deposition. This requires submergence of the substrate in an appropriate plating bath, and of course the plating mask must be sufficiently robust to withstand exposure to the plating bath. If, after electroless plating of a portion of the desired metal pattern, electroplating is used for depositing the remaining portion, the process is known as the semi-additive process.
As electronic systems become more complex and more miniaturized, there is a continuing demand for higher circuit density, and for more closely packed and more precisely defined conductors. Part of this demand can be met by using multilayer PWBs in which a plurality of substrates, each having a metal pattern defining part of the electronic circuit, are laminated together. Conductive interconnection between the PWB layers is made by holes through adjoining layers known as conductive vias. Nevertheless, there has been a continuing demand for conductors that are sufficiently thick to conduct the current required, and yet are made with such high precision and definition as to permit closer conductor spacing.
With the subtractive process, it has been found to be difficult to obtain the required high precision because of undercutting of the metal layers by the etch. With the additive or semi-additive process, it has been found to be difficult to form plating masks that are sufficiently thick to match the thickness of the plated metal, sufficiently robust to withstand exposure to a plating bath, and which still can be patterned with the requisite precision.